An electrically variable transmission (EVT) can be advantageously used in conjunction with an internal combustion engine to provide an efficient parallel hybrid drive arrangement. Various mechanical/electrical split contributions can be effected to enable high-torque continuously variable speed ratios, electrically dominated launches, regenerative braking, engine off idling, and multi-mode operation. See, for example, the two-mode, compound split, electromechanical transmission shown and described in the U.S. Pat. No. 5,931,757 to Schmidt, where an internal combustion engine and two electric machines (motors/generators) are variously coupled to three interconnected planetary gearsets. Such parallel EVT transmissions enjoy many advantages, such as enabling the engine to run continuously in high efficiency operating regimes, and achieving both reverse and forward operation in the same mechanical mode simply by controlling the rotation of the electric machines.
In a typical application, the electric machines are controlled for achieving both a desired input (i.e., engine) speed and a desired output torque. A similar control can be used in the case of mechanical mode changing (i.e., shifting), in which case the control can be based on a desired clutch slip profile. Various measured or estimated input parameters for the control include the transmission input torque and the input and output shaft speeds and accelerations. While the control objectives may theoretically be achieved with an open-loop control strategy, some form of closed-loop correction is usually employed in order to compensate for modeling and estimation errors. For example, a closed-loop correction may be determined based on input speed error, and attributed to the most likely sources of error. However, errors may also arise due to unexpected circumstances such as sub-system abnormalities and apparent changes in inertia due to friction, and the authority of the closed loop correction may be insufficient to compensate for the errors. In such cases, the actual output torque may deviate from the desired output torque, and it would be desirable to detect the existence of such a possibility, particularly at low speed operation of the transmission near the boundary between forward and reverse operation.